Epitaxial growth of GaN on Si substrates has recently gained increased interest since such a system presents possibilities for novel integrated devices based on GaN and Si [1-3] at reduced cost. This is particularly the case for LEDs which are traditionally low cost items and are required in large numbers to meet demand in display and lighting applications. Such a system would provide the potential for utilizing the strength of GaN in conjunction with advanced Si technology and abundance for effective and economical integrated device structures. However, the large misfit between GaN and Si (14% between a-axes), growth of a polar crystal on non-polar substrates and the difference in thermal expansion coefficients lead to a high density of lattice defects and antiphase disorder as observed in GaAs grown on Si [4]. Due to a lack and/or prohibitively high cost of native substrates for growth of GaN, this material needs to be grown on foreign substrates such as Al2O3 or SiC. Therefore, different approaches need to be applied, such as lateral overgrowth and pendeo-epitaxy, to reduce defect density in the epi-layer [5-10].
Reduction of strain at the interface leads to a lower defect density. Earlier results [11-12] show that H and He implantation through a pseudomorphic Si—Ge layer grown on (100) Si substrates below critical layer thickness and subsequent annealing at 850° C. leads to complete strain relaxation and defect free Si—Ge layers in comparison with unimplanted samples. The genesis of defect-free growth lies in the fact that implantation energy was chosen to create end of range defects slightly below Si—Ge/Si interface (50-100 nm). The H or He bubbles are formed in Si following annealing. A much denser arrangement of tangled misfit dislocations was found at the interface between the Si1-xGex (x=22% -30%) and Si substrates in comparison with regularly distributed misfit dislocations grown on un-implanted Si, where only 50% relaxation was obtained upon annealing at 1100° C. A model was proposed [11] for strain relaxation due to the formation of dislocation loops in the vicinity of He bubbles that annihilate with threading dislocations at high temperature when the loops become glissile and can glide toward the SiGe interface. It has been proposed that one side of the loop is pinned at the interface where it forms a strain relieving misfit segment. The other side is driven by the mismatch stress to the surface, where an atomic step is generated. In the case of GaN however, pseudomorphic growth on Si is impossible due to the lattice misfit being too large.
Different approaches are therefore needed to permit epitaxial growth of GaN on Si substrates without threading dislocations since pseudomorhic growth of GaN on Si is impossible due to large lattice and thermal expansion mismatch.